Heretofore, to ensure safety when using motor driven apparatus, it has been necessary to have a detection device which can detect a sudden unintentional start up of a stopped motor and give a warning.
An example of this is the monitoring to ensure that a movable portion of a robot does not suddenly move due to electrical noise when the robot is in a hold condition (a temporary stop condition instructed by the controller program) as reported by Sugimoto et al. in; Transactions of the Japan Society of Mechanical Engineers Vol. 56-C, No. 530 (1990-10), and also disclosed by K. Futsuhara, N. Sugimoto, M. Mukaidono et al. in; Proceedings of the Second International Conference on Human Aspects of Advanced Manufacturing and Hybrid Automation, Honolulu, U.S.A. (August 1990) in a paper entitled, "A Method of Constructing Safety Device with Consideration of Noise-Induced Errors". Also at the same conference in Honolulu, M. Kato et. al proposed in a paper entitled "Construction of Magnetic Sensors for Assuring Safety", a method for verifying that motor rotation has stopped, by monitoring for current flowing in the motor during the robot hold condition.
Moreover, the 1959 Japan Industrial Robot Manufacturers Association Report points out that such accidents have actually occurred, and there is concern that these are on the increase.
With the apparatus for verifying that motor rotation has stopped disclosed in the abovementioned papers, with the case of M. Kato et. al, it is stated that, "when electricity is supplied to the motor, this is equivalent to when the motor is rotating[?]", while in the paper of N. Sugimoto et. al., a mechanical construction is adopted wherein an arm is attached to a rotation shaft of the motor, and any movement of the arm while the motor is stopped is detected by means of a limit switch.
The former case has a weakness in that the rotation of the motor cannot actually be detected when the rotor (rotating element of the motor) is rotating under inertia, with the power supply to the motor shut off.
Moreover, with the latter case, in order that when the motor is running, the limit switch is not operated by movement of the arm, the limit switch must be made to operate under conditions that the motor is stopped. Such an arrangement wherein the limit switch is operated as soon as the motor is stopped requires prior provision of a function which can detect stoppage of the motor. This means that some device for verifying that the motor has stopped must be specially arranged beforehand.
With regards to the above, it becomes apparent that, as a safety measure when using motor driven apparatus, it is essential to have an apparatus for reliably determining that motor rotation has stopped (including rotation due to inertia).
The present invention takes into consideration the abovementioned situation with the object of providing a motor rotation judgement circuit which can judge motor rotation when the motor is rotating under inertia as well as when rotating with power supplied thereto, and distinguish this from a motor stopped condition, and which can reliably judge when motor rotation has stopped, and also which has a fail-safe construction which at the time of a failure can produce a judgement output the same as for when the motor is rotating. An object of the second aspect of the invention is to provide a motor stopped verification apparatus which uses such judgement circuit for verifying that the motor has stopped rotating.